This invention relates generally to a method and apparatus for monitoring a process being performed to transform an article in some way, particularly an exposed surface of the article. In a specific form, this invention more particularly relates to a technique for detecting an endpoint of a chemical-mechanical-polishing ("CMP") process on a surface carried by a semiconductor wafer.
CMP, usually resulting in planarization of a surface, is a key technology used for the fabrication of advanced microelectronic devices (e.g. those having critical dimensions that are less than about 0.5 micron) on substrates. Planarization allows smaller features to be fabricated, which leads to denser circuit packing and faster circuit response, which in turn results in faster logic devices and higher capacity memories. In addition, planarized surfaces allow for the fabrication of circuit devices having multiple levels of metallization, and therefore a more sophisticated design. The process consists of mechanically grinding or polishing of the work piece (generally a thin film microelectronic circuit imposed on a silicon wafer) against a rotating polishing wheel (platen), in the presence of a chemical slurry therebetween. The slurry provides both abrasive particles (to accomplish the mechanical grinding) and reactive chemicals (to assist the film removal by chemical modification of the film surface). The wafer is typically held (by frictional forces) to a polishing head or wafer carrier, which itself rotates, and which provides downward pressure of the wafer against the platen. A porous polymeric polishing pad mounted on the platen helps to trap the abrasive slurry particles, and provides a conformable surface that aids in the planarization process.
Process control, and specifically endpoint control, has become a key issue in CMP, in part because of process variations, such as in the above mentioned slurry and polishing pad, and in part because of variations in the thickness, composition, and uniformity of the films to be polished. The Prior Application referenced above is directed to such process control. Briefly, the technique mentioned in the Prior Application requires directing through the back side of the wafer to be polished, an interrogating optical beam having a wavelength to which the wafer is substantially transparent. The state of removal of the film on the front side of the wafer is then inferred from the time evolution of the amount of the interrogating light which is reflected from the film surface. In the case of opaque (e.g. metal) films, a pronounced shift in reflected light intensity often occurs when the film has been completely removed, thus signaling an endpoint of the process. In the case of transparent films (e.g. silicon dioxide), the time evolution generally consists of alternating intensity peaks and valleys caused by the well known effects of thin film optical interference. By analyzing the pattern of peaks and valleys, one can determine the amount of film that has been removed, thus allowing an endpoint of the process to be called when a predetermined amount of film removal has occurred.
Although the techniques described in the Prior Application are quite useful in monitoring and controlling the CMP process, further improvements and implementations of the techniques with actual CMP equipment are desirable. Accordingly, it is a primary object of the present invention to improve the mechanical and electrical connections of the film sensing apparatus, so as to allow ease of installation, replacement, and service.
It is another object of the present invention to provide techniques for monitoring and controlling the CMP process, and to provide a measure of film removal uniformity across the film being treated.
It is a more general object of the present invention to provide techniques for monitoring and controlling various processing steps, in addition to CMP, used in manufacturing integrated circuits, flat panel displays, and the like.
It is yet another object of the present invention to utilize these same techniques for monitoring and controlling the processing of other types of articles as well.